The mechanism of cyclization of uridine-3(-p-nitrophenyl phosphate catalyzed by various transition metals has been studied using heavy atom isotope effects. The positions where relevant isotope effects are measured are the nitro nitrogen and all phosphate oxygens. The isotope effect of the nitro nitrogen has been measured with several different transition metal catalysts and the data indicate significant delocalization of electrons into the ring in the transition state. The synthesis of compounds isotopically labeled at the phosphate oxygens is underway. These compounds will be used to determine the oxygen isotope effects, which will provide additional insight into the structure of the transition state of phosphate cyclization. This will allow the mechanism of these reactions to be better understood.